writefln on interface array

[strtr]

Is it possible to have this output [null,"1"] in stead of Error: std.format 
formatArg?

interface I{}
class C:I{
int index;
char[] toString(){ return toString(index) }
}

I[2] two_i;
I[1] = new C();
writefln(two_i);



Would be handy for debugging ;)